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A hiding place for the Earth’s missing xenon

28 April 2014

A collaboration between
researchers in Jilin, China and University College London, including Professor
Chris Pickard of the Thomas Young Centre and the Department of Physics and
Astronomy, has identified a possible resting place for the Earth’s elusive
store of noble xenon.

The atmosphere of the
Earth contains far less xenon (the second heaviest noble gas) than expected. It
is one of the enduring mysteries of the planetary sciences and is often
referred to as the “missing xenon paradox”.

Combining advanced
structure prediction techniques, and high quality quantum mechanical
calculations, the research team have shown that at the pressures found in the
Earth’s core both iron and nickel (its main constituents) react readily with
xenon to form a variety of thermodynamically
stable compounds - most importantly the cubic Fe3Xe (see Figure). Using diamond anvil cells, the conditions at the
centre of the Earth can be reproduced in the laboratory. Experiments which have
tried to make iron xenon compounds have never been successful, and earlier
calculations made assumptions about the likely structures and found no bonding.
It had been concluded that iron and
xenon do not react in the Earth’s core.

Advances in structure
prediction (such as ab initio random structure
searching, developed by Prof Chris Pickard, and swarm based approaches
pioneered by Jilin researcher Prof Yanming Ma) allow the unbiased discovery of
unsuspected materials. New, more stable, xenon iron compounds were found in the
current study, and the earlier conclusion overturned. Awaiting experimental
confirmation of the result, their article published in Nature Chemistry
proposes that the Earth’s core is a natural hiding place for the missing xenon.

Figure:Cubic Fe3Xe. At sufficiently high
pressure (around those found in the Earth’s inner and outer cores) the normally
inert xenon reacts readily with iron and nickel.